Pa
Protactinium
Element 91 · Actinide
Protactinium (Pa)
Protactinium is a rare, toxic, radioactive metal. It occurs in tiny amounts in uranium ores.
- Actinide
- Period 7
- f-block series
- f-block
- solid at room temperature
- ☢ Radioactive
Key properties
- Atomic mass
- 231.0359
- u
- Electron configuration
- [Rn] 5f2 6d1 7s2
- Melting point
- 1,571.9 °C
- 1,845 K
- Boiling point
- 4,026.9 °C
- 4,300 K
- Density
- 15.37 g/cm³
- Electronegativity
- 1.5
- Pauling
- Atomic radius
- 243 pm
- van der Waals
- 1st ionisation energy
- 5.89 eV
Compare with every element:Electronegativity chartAtomic radius chartIonisation energy chartDensity chart of the elementsMelting point chartElectron configuration chart
Did you know?
Notable facts about protactinium
- 01
Lise Meitner and Otto Hahn identified the long-lived isotope protactinium-231 in 1917–18.
- 02
Of all the elements found in nature, it ranks among the scarcest and costliest.
- 03
Protactinium–thorium ratios are used to date ocean sediments.
Applications
What is protactinium used for?
- Ocean sediment dating
- Scientific research
History
Discovery & name origin
Discovered by
Year
Where
Germany
Name origin
Greek protos, “first” + actinium — it decays into actinium.
Atomic structure
Atomic properties
Atomic structure
Atomic properties
- Atomic number
- 91
- Atomic mass
- 231.0359 u
- Group
- Actinides (3)
- Period
- 7
- Block
- f-block
- Electron configuration
- [Rn] 5f2 6d1 7s2
- Full configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f2 6d1
- Electrons per shell
- 2, 8, 18, 32, 20, 9, 2
- Electronegativity (Pauling)
- 1.5
- Electron affinity
- —
- Atomic radius (van der Waals)
- 243 pm
- Stable isotopes
- None — all isotopes are radioactive
Oxidation states
Highlighted: common oxidation states (+5, +4).
Physical
Physical properties & colour
Physical
Physical properties & colour
- State at 20 °C
- Solid
- Melting point
- 1,845 K · 1,571.9 °C · 2,861.3 °F
- Boiling point
- 4,300 K · 4,026.9 °C · 7,280.3 °F
- Density
- 15.37 g/cm³
- Molar heat capacity
- —
- Appearance
- Bright, silvery metallic lustre
- CPK model colour
- Copied
The CPK colour is the convention used for this element’s atoms in molecular models.
FAQ
Protactinium questions
What is Pa on the periodic table?
Pa is the symbol for protactinium, element 91 on the periodic table. It sits in period 7, in the f-block, in the actinide category, and has an atomic mass of 231.0359.
What is the atomic number of protactinium?
Protactinium has atomic number 91, so every protactinium atom has 91 protons in its nucleus. Its symbol is Pa and its atomic mass is 231.0359.
Who discovered protactinium?
Protactinium was discovered by Kasimir Fajans and Oswald Helmuth Göhring in 1913 (Germany). The name comes from Greek protos, “first” + actinium — it decays into actinium.
What is protactinium used for?
Protactinium is used for: ocean sediment dating; scientific research.
Is protactinium a metal?
Yes. Protactinium is a actinide, one of the metallic elements.
What is the electron configuration of protactinium?
The electron configuration of protactinium is [Rn] 5f2 6d1 7s2. Its 91 electrons are arranged in shells as 2, 8, 18, 32, 20, 9, 2.